Improve docs on Gradle User Home caching

- Describe the limitations/properties of the GitHub Actions cache
- Document the algorithm for generating a cache key, and the way that cache entries are matched
- Describe in more detail how entries are de-duplicated
- Explain how cache entries can be optimized in Job pipelines

Fixes #831
Fixes #608

src/cache-utils.ts

@@ -66,7 +66,7 @@ export class CacheKey {
  * - The cache protocol version
  * - The name of the cache
  * - The runner operating system
- * - The name of the Job being executed
+ * - The name of the workflow and Job being executed
  * - The matrix values for the Job being executed (job context)
  * - The SHA of the commit being executed
  *